Traditionally, crankcases or engine blocks may be manufactured for internal combustion engines from aluminum or aluminum alloys in a high pressure die casting (HPDC) process. The HPDC process may provide weight reduction and heat transfer enhancements relative to cast iron materials.
For meeting the tribological requirements, it is also known to use cylinder liners consisting of gray cast iron material with a wall thickness of typically between 2 and 4 mm in engine blocks consisting of aluminum or aluminum alloys. In this case, some of the advantages with regard to the dissipation of the waste heat are lost since the thermal conductivity of gray cast iron material with approximately
  40  -      50    ⁢                  ⁢          W              m        ·        K            is only a fraction of the thermal conductivity of the aluminum material or approximately
  140  ⁢          ⁢            W              m        ·        K              .  
Therefore, cylinder liners consisting of high thermal conductive aluminum are also used. From the article of K. Bobzin, F. Ernst, K. Richardt, T. Schlaefer, C. Verpoort, and G. Flores: “Thermal spraying of cylinder bores with the Plasma Transferred Wire Arc process” in Surface and Coatings Technology, Vol. 202, Edition 18, Jun. 15, 2008, p. 4438-4443, it is known that engine blocks of automobiles consisting of sub-eutectic AlSi alloys are customarily equipped with cast iron sleeves in order to obtain cylinder bore surfaces which satisfy the tribological requirements. Thermally sprayed cylinder bore surfaces are described therein as a promising alternative to gray cast iron liners. Atmospheric plasma sprayed (APS) cylinder bore surfaces consisting of low-alloyed C-steel had already proved their capability to reduce friction losses in engines. Additional potential for reducing friction losses is unprecedented and is attributed to high-alloyed surface materials on an iron base. The article describes the development of such materials and their use via the thermal plasma transferred wire arc coating (PTWA) process on inner walls. The feed materials lead to partially amorphous coatings with embedded boridic, nanoscale precipitations if they are processed by thermal spraying. The coatings were deposited on the inner walls of test liners consisting of aluminum EN AW 6060 and on the cylinder bore walls of a 4-cylinder inline engine. Before coating, all the surfaces to be coated were pretreated by a new type of fine boring process in order to create a surface topography which enables the adherence of the coatings. The microstructures of the coatings were analyzed via optical microscopy, durometry and transmission electron microscopy, and the oil retention capacities of the honed surfaces were determined.
In other alternative approaches, the use of cylinder liners is dispensed with, and the cylinder walls of the engine block are coated in order to achieve for example the desired resistance to friction and wear. The coatings are designed in respect to material choice and arrangement depending on the desired function.
For producing the coatings, thermal processes are used, wherein particular attention is to be paid to a trouble-free application of the coating on the cylinder wall which is to be coated. In previous examples, specific processes and devices are proposed for this.
For example, WO 2016/202511 A1 describes a thermal spraying method and a device for coating the inner surface of a cylinder of an internal combustion engine or piston engine, wherein the method features applying a thermal spray layer to the inner surface of the cylinder and optical detection of the surrounding of the spray jet, specifically of a space outside the spray jet, via an optical sensing device. In this case, an error in the coating process is assumed if particles of the spray material, which is fed to the spray burner, are detected by the optical sensing device in the monitored region outside the spray jet. For example, the thermal spray process is formed by known plasma transferred wire arc spraying (PTWA) processes or rotating single wire (RSW) processes.
Proposed in DE 10 2017 103 715 A1 is a coating of a cylinder liner or cylinder wall with a functional layer which on account of its variable porosity ensures different lubricating requirements in different regions of the cylinder bore are fulfilled.
The engine block, which for example can be produced from cast iron, aluminum, magnesium or alloys thereof, can have a body which has at least one cylindrical engine bore wall with a longitudinal axis, and has a variable coating, extending along the longitudinal axis, which has a coating thickness. The coating can have a middle region and a first and a second end region, and a plurality of pores can be distributed in the coating thickness. The middle region can have a different average porosity than one or both of the end regions. The method can involve thermal spraying of a coating with a first porosity in a middle longitudinal region of the bore and spraying of a coating with a second porosity in one or more end regions of the bore. The coating can be all coatings which provide sufficient mechanical strength, rigidity, density, wear properties, friction, fatigue strength and/or thermal conductivity for a cylinder bore, and can especially also be formed by a coating with iron, steel, other metals or non-metals, as a ceramic coating, polymer coating or as an amorphous carbon coating. The first porosity can be greater than the second porosity, and the first porosity and the second porosity can be formed during the spraying step. One or both of the end regions can have an average porosity of between 0.1% and 3%. The middle region can have an average porosity of at least 5%. The pores can act as recesses for lubricant in this case, as a result of which lubrication under rough operating conditions is provided and the lubricant film thickness is improved.
The application of coatings on cylinder walls for influencing heat flows during operation of the internal combustion engine is also known.
For example, EP 3 228 852 A1 proposes an internal combustion engine with a combustion chamber, which is enclosed by at least one inner wall of a cylinder bore, a cylinder head, a valve and a piston, and a coating layer which is arranged on at least one part of the inner wall of the combustion chamber via a flame spraying process, wherein the thermal conductivity of the coating layer at room temperature is lower than the thermal conductivity of the cylinder block, the cylinder head, the valve and the piston. In this case, the thermal conductivity of the coating layer, which for example can contain a quasi-crystalline metal alloy, especially an Al—Cu—Fe-based alloy, or a metallic glass, is reversibly increased with a rise of the temperature of the coating layer, and the thermal capacity per unit area of the coating layer is greater than
  0  ⁢      kJ                  m        2            ·      K      and less than, or equal to,
  4.2  ⁢            kJ                        m          2                ·        K              .  As a result, the effect of minimizing cooling losses of the combustion chamber and consequently the fuel consumption is to be achieved and at the same time knocking of the internal combustion engine can be mitigated.
Described in JP 4812883 B2 is a cylinder liner for insert casting and for use in a cylinder block consisting of an aluminum alloy, wherein a layer with a thermal conductivity which is lower than a thermal conductivity of at least one out of the cylinder block and the cylinder liner is formed by an intermediate section of the cylinder liner in the axial direction toward a lower end. The layer can for example consist of a sprayed-on layer of ceramic material; in this case aluminum oxide is used as the ceramic material. The layer is formed via thermal spraying, for example, via plasma spraying or high-velocity oxygen fuel spraying (HVOF). As a result of the low thermal conductive layer, there should be the possibility of preventing a temperature at the lower end of the cylinder liner dropping undesirably low during operation of the cylinder block, which can lead to increased viscosity of the lubricating oil and therefore to higher fuel consumption.
Proposed, moreover, in JP 2016205215 A is a method for producing a cylinder block which has a higher thermal conductivity coefficient on an outer circumferential wall of a cylinder bore on an upper part than that of a lower part of the cylinder bore in its axial direction without any complex steps having to be applied for establishing cylinder liners with a different thermal conductivity coefficient in an axial direction on a casting mold for the cylinder block. In the method, a cylinder bore is designed with a standard inside diameter by forming a cylinder-block main body, for example from an aluminum alloy, in a way in which an inside diameter is created on a lower part of a bore hole for forming the cylinder bore which is larger than an inner diameter of an upper part of the bore hole. After that, material with low thermally conducting material, for example an iron-based material, with a lower thermal conductivity coefficient than that of the material forming the main body of the cylinder block, is flame-sprayed against a first circumferential wall surface and a second circumferential wall surface of the circumferential surface, which defines the bore hole, of the cylinder block main body in order to form a sprayed layer, wherein the sprayed layer on the first circumferential wall is thicker than the sprayed layer on the second circumferential wall.
Furthermore, a cylinder liner, for example consisting of cast iron and for insert casting, which is used in a cylinder block consisting of an aluminum alloy, is known in U.S. Pat. No. 7,685,987 B2. The cylinder liner has an outer circumferential surface and upper, middle and lower sections with regard to an axial direction of the cylinder liner. A high thermally conducting layer, for example consisting of an aluminum-silicon alloy, is formed in a section of the outer circumferential surface which corresponds to the upper section, and a low thermally conducting layer is formed in a section of the outer circumferential surface which corresponds to the lower section. A sprayed-on layer mainly consisting of a ceramic material such as aluminum oxide and zirconium oxide can be used as material of the low thermally conducting layer. Alternatively, the low thermally conductive layer can be formed from a sprayed layer of a material on an iron base which contains oxides and a number of pores. The high thermally conductive layer and the low thermally conductive layer are laminated in a section of the outer circumferential surface which corresponds to the middle section, as a result of which a laminated layer section is formed. As a consequence of this, the temperature difference along the axial direction of the cylinder is reduced, as a result of which the fuel consumption can be reduced.
In light of the illustrated previous example, the field of coating piston paths of internal combustion engines, which are arranged either on the inner wall of a cast cylinder liner in the engine block or on the inner wall of a cylinder bore of an engine block, especially of internal combustion engines having engine blocks consisting of aluminum or at least an aluminum alloy, still provides room for improvements with regard to a thermal design and enhancement of waste heat flow.
In one example, the issues described above may be addressed by an engine block comprising a first coating arranged on interior surfaces of a cylinder near a top-dead center position of a piston and a second coating arranged on the interior surfaces near a bottom-dead center position of the piston, the first coating comprising a hypereutectic aluminum-silicon alloy and the second coating comprising an iron-based alloy with a thermal conductivity lower than the first coating and the interior surfaces. In this way, thermal conductivity in the combustion chamber may be enhanced to promote heat dissipation or heat retention as desired.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.